1. Field of the Invention
The present invention relates to a constant current circuit which generates a constant current.
2. Description of the Related Art
In general, a constant current circuit is widely used in a semiconductor integrated circuit. Characteristics of the constant current circuit are important factors in determining the performance of the semiconductor integrated circuit.
FIG. 4 illustrates a configuration of a conventional constant current circuit 100 in prior art. The constant current circuit 100 includes a current mirror circuit 101 constituted of transistors M1 and M2 each being a P-channel MOS transistor, a current mirror circuit 102 constituted of transistors M3 and M4 each being an N-channel MOS transistor, and a resistor R1 (refer to, for example, Japanese Patent Application Laid-Open No. 2009-193211). The resistor R1 is interposed between a source of the transistor M3 and a conductive line LVSS for a power supply voltage VSS.
Further, a startup circuit 103 supplies a prescribed current from an internally provided current source to the transistor M1 to supply a mirror current to the current mirror circuit 101, thereby starting up the constant current circuit.
As described above, the constant current circuit is constituted as a current mirror circuit which supplies a reference current corresponding to the mirror current flowing through the transistor M1.
There has been a demand that the current mirror circuit compensates variations in the transistor and the like due to a manufacturing process and continually supplies a constant reference current.
In the constant current circuit according to Japanese Patent Application Laid-Open No. 2009-193211, however, at an occurrence of fluctuation of the power supply voltage VDD applied to the conductive line LVDD, the constant current fluctuates in a transient state corresponding to the fluctuation in the power supply voltage VDD since transient current flows through parasitic capacitances between each terminal of the transistor and the conductive lines.
That is, in the constant current circuit, since the transient mirror current increases upon rising of the power supply voltage VDD from a prescribed voltage, the constant current is held at an increased state during a period corresponding to this increase. On the other hand, since the transient mirror current decreases upon lowering of the power supply voltage VDD from the prescribed voltage, the constant current is held at a decreased state during a period corresponding to this decrease.